Bistable device employing threshold gate circuits



p 7, 1966 c. L. COATES, JR., ETAL 3,275,849

BISTABLE DEVICE EMPLOYING THRESHOLD GATE CIRCUITS Filed Nov. 8, 1963 5 Sheets-Sheet 1 OUTPUT 0 ourpur L f a 5 MB 4 0 Q 6 OUTPUT Fcr /2 a We l 'MAQ/ /0= Invento r's:

+ V/ Clarence L..Codt'es c/fi;

Philip M Lewis: I.

p 6' c. L. COATES, JR., ETAL 3,275,849

BISTABLE DEVICE EMPLOYING THRESHOLD GATE CIRCUITS Filed Nov. 8, 1963 5 Sheets-Sheet 2 Inventor's: C/dr-ervce L.. Cod-fies dr-y phi ip M Lewis 2'.

he/PAttor'rvey.

Sept. 27, 1966 r c. LICOATES, JR, ETAL v 3,275,849

BISTABLE DEVICE EMPLOYING THRESHOLD GATE CIRCUITS Filed Nov. 8, 1963 5 Sheets-Sheet 5 our I 9' ITV M a Mil: 1

Inventors:

C/arence L. Codfies mg phi/l'p M Lew/s11,

heir" Attorney.

P' 1966 c. L. COATES, JR, ETAL 3,275,849

BISTABLE DEVICE EMPLOYING THRESHOLD GATE CIRCUITS Filed Nov. 8, 1963 5 Sheets-Sheet 5 [nve n t or's: Clarence L. Codtes Jr; Philip M. Lewis 27., by PM e/r- Attorney.

United. States Patent 3,275,849 BISTABLE DEVICE EMPLOYING THRESHOLD GATE CIRCUITS Clarence L. Coates, Jr., Austin, Tex., and Philip M.

Lewis II, Schenectady, N.Y., assignors to General Electric Company, a corporation of New York Filed Nov. 8, 1963, Ser. No. 322,349 Claims. (Cl. 307-885) This invention relates to a bistable device and particularly to such a device including threshold gating elements.

A threshold gating element isa circuit element providing an electrical output when the summation of applied electrical inputs exceeds a predetermined value. Computing devices including threshold elements in logical circuits have the advantage of considerably reducing the number of components required as compared to prior computing circuitry using more conventional and or" logic gating elements, inasmuch as each new input to a threshold gate requires only an extra input resistor. In addition to the logical gating function, :a complete computer, however, requires storage, wherein data provided by or for the computer is held until needed. It is a purpose of the present invention to combine the logical and storage functions in a bistable circuit formed of threshold gating elements. This device not only tests the inputs provided thereto on a logical basis by comparison with a predetermined threshold, but remembers or sets upon the last input condition.

According to the present invention, a pair of complementing threshold logical elements are cross-coupled whereby one triggers the condition of the other in a bistable manner. Since each logical threshold element is complementing in nature, one logical element of a pair will be energized at any given time. If either gate is in the off or deenergized position, an output is provided which forces the opposite gate into the energized condition.

Generally, similar logical inputs are applied to the two gates of a bistable pair. In accordance with one aspect of the present invention, inputs supplied to one of the gates of a pair'may include complements of the inputs supplied to the other gate. Thus the logical criterion for operating only one gate at a time is met by the inputs.

The subject matter which we regard as our invention is clearly pointed out and distinctly claimed in the con cluding portion of this specification. The invention, however, both as to organization and method of operation together with further objects and advantages thereof may be best understood by reference to the following description taken in connection with the accompanying drawings wherein like reference characters refer to like elements and in which:

FIG. 1 is a basic representation of a threshold gate element,

. FIG. 2 is a basic representation of a complementing gate element, 7

FIG. 3 is a schematic diagram of the threshold element of FIG. 2,

FIG. 4 is a generalized logical diagram of a bistable device in accordance with the present invention,

FIG. 5 is a logical diagram of a first exemplary bistable device in accordance with the present invention,

FIG. 6 is a logical diagram of a second exemplary device in accordance with the present invention,

'FIG. 7 is a logical diagram of a third exemplary device in accordance with the present invention,

FIG. 8 is a schematic diagram of the FIG. 5 device,

FIG. 9 is a schematic diagram of the FIG. 6 device, and

FIG. 10 is a schematic diagram of the FIG. 7 device.

3,275,849 Patented Sept. 27, 1966 A threshold element as utilized in accord with the present invention produces a binary one when a function of its plurality of the indicated binary inputs exceeds a preset threshold value. Such a threshold element or gate may be represented as in FIG. 1 by a circle enclosing a ratio, U:L, of numbers which indicates the threshold gap. The actual value of threshold is advantageously set to be within this gap. For the threshold element as illustrated in FIG. 1, having a gap equalling 4:3, four or more inputs of value one are required to produce an output, but three such inputs produce no output. Weights are given the inputs as indicated by numbers included in the input leads. A weight of two doubles the input so it has the same effect as two inputs of unity Weight. Therefore one input of weight two, plus two inputs of weight one, will operate the gate.

In the gap ratio, the first number U, is the smallest value of the summation of weighted inputs, za x for which m x, exceeds the gate threshold, and L is the largest value for which za x is below the threshold. In the expression, 2am, x indicates an input in the it]:l order digit position, and a indicates the weight applied to such input. The tolerance or allowable variation in component values (and threshold value) is related to the gap, and can be shown to equal U-L/U+L. It is apparent that a smaller gap results in a smaller or tighter tolerance in component values.

FIG. 1 illustrates a straight non-complementing gate.

FIG. 2 illustrates a complementing gate indicated by the bar over the gap numerals. When the sum of the inputs, za x applied to this gate exceed the threshold, the

gates normally-on output is interrupted. Thus an in- 'verse output is produced. In the FIG. 2 illustration, the inputs are also shown as complements. That is, an input signal current is applied to the particular input lead in the absence of the quantity given.

An exampleof a circuit diagram for a typical threshold gate element is illustrated in FIG. 3. The gate illustrated here is inherently a complementing threshold element; that is, a transistor amplifier connected in this manner produces an inverted output. For illustrative purposes, the FIG. 3 threshold gate is taken as an embodiment of the gate shown schematically in FIG. 2, having the same inputs and input weights.

The circuit illustrated in FIG. 3 produces a negative output voltage, taken as a binary one, when the summation of negative inputs, Ea x does not exceed the threshold setting. No output, or a binary zero is produced when the summation of negative inputs, 2am, exceeds the threshold setting. The circuit comprises a transistor amplifier of grounded emitter configuration having a plurality of equal valued input resistors 1-7 connected between the input terminals and the transistor base 8. The input signals are taken as negative going in each instance. The resistors 1-7 have the same resistance, individually providing unit weight to negative polarity inputs applied thereto. Thus resistors 1, 2 and 3- provide unit weight for inputs designated 0, P and Q. However, resistors 4 and 5, connected in parallel, provide double unit weight to input hi, and they parallel combination of resistors 6 and 7 likewise provide double unit weight to M, since a doubled flow of current may take place through the paralleled resistors in each case.

In the absence of sufficient input for exceeding the threshold, a diode 10 prevents base 8 from rising above ground level, while transistor collector 14, supplied a negative voltage -V through resistor 11, is similarly prevented from becoming more negative than a voltage V;, by means of clamping diode 13. Thus the gate normally supplies a negative output current derived through the diode 13.

I rises. to near zero.

x given respective weights a a;

A threshold resistor 9, which may be made conveniently 7 variable, couples transistor base 8 to. a source of positive voltage V This resistor is used to determine the threshold of conduction for the transistor. The threshold .resistor 9, jointly with input resistors 1-7, comprise a voltage divider having a midpoint at the transistor base 8.

In the absence of the prescribed summation of gate inputs required for exceeding the threshold, the voltage drop across threshold resistor'9 is insutficient to lower the. transistor base from ground potential. However, when a number of inputs occur exceeding the preset threshold,

these inputs collectively provide sufficient current through their respective input resistors for swinging the transistor base below ground and operate the transistor under saturation conditions. At this time, maximum collector emitter current flows in the transistors establishing 21 voltage drop across load resistor 11 whereby output terminal 12 rises-to a low value nearsground level. Th-usthe output terminal:12 supplies the voltage equalling minus V until the threshold is exceeded, atJWhich time the output voltage This particular circuit is therefore a complementing threshold circuit, as indicated, since an output is produced in the absence of inputs exceeding the gate threshold.

FIG..4 is a logical diagram of a bistable device according to the present invention including a first complement ing threshold gate 15 and a second complementing threshold gate 16 having gaps U :L and U :L respectively. In general it is convenient, but not necessary, for these gaps to be equal. The first complementing threshold element 15 receives a number, of logical inputs x x a Similarly, igateelement 16 receives inputs y y weights b b b Since gating element 15 is' a I complementing gate, an output 1 is produced on output lead 17 if the summation of weighted inputs does not exceed the threshold. An output 0 is produced when 7 they do.

Cross-coupling lead 18 connects output lead 17 as a feedback input to gating element 16, Where it receives a weight of b Gating element 16, producing an output, on lead 19, in the absence of weighted inputs exceeding the gates threshold, is cross-coupled via feedback lead 20 to an inputof gating element 15. This cross-coupled sigputs on the gate whose output is 1 are such as to exceed its threshold, while the logical inputs on the other gate do not exceed its threshold. Under these conditions the output of thefirst gate goes to zero, allowing the output of the other gate to go to 1,.which in turn keeps the first gate at zero. I

It is frequently desirable, for some or all of the logical inputs applied to one threshold gate to be complements y having and TN, are data inputs and W or Write is denominated a control input for gating data into the bistable device. v

Gate 15 having a gap, receives. the. signal IN with a weight of. 1..! Gate 16 also having a gap .of 2:1 receives IN with a weight of 1. The control input Wisapplied to both gates with aweight ofl. Feedback connections 18 i and '20 are eachgiven a weight of 3, :sufiicient for .exceeding the threshold of each gate- This heavy weighting, greater thannecessary toexceed the threshold, improves the state-changing-time-constant of the circuit. i

To store a binary digit-fin the FIG. 5 device, IN is set equal to a binary input value x which may be zero 0r onqfi'is setequal to 1?,3 and ;Wis made concurrently equal to 1.. Then the quantity. fx" is entered into, the device for storage therein. a The state. of. the. bistable device is not affected eitherif W isequal to zero,or.if

W is equal to 1 and IN and F are both equal to zero.

1 The operation of the circuit of FIG. Scan then be characterized by the Boolean expression: W'-IN,'that is. W and IN. Thus, the quantity W, in combination withthe input, IN, gates the inputsignal, from IN, into .the .bistable device, or sets this irfput signalinto .thedevice.

In FIG. 6, W is again the control input, and INi, IN, IN] and IE are data inputs. As long as W=0, thedata inputs can have any value without setting the bistable deof logical inputs applied to theothergate. of :the pair. 4

Then, if the threshold of one gate is'exceeded, the threshold of the .other gate isnot exceeded, since the presence of a particular input quantity dictates the absence of its complement. 'In certain instances it is also desirable,

but not necessary, that any combination of inputs applied determines the state of the bistable circuit. That isto say, the threshold of one gate or the other will then always be exceeded, as a consistentarray of input information is applied to the pair of gates.

Various specific embodiments of the invention are ill-ustrated in FIGS. 5, 6 and 7. In FIG. 5 the inputs, IN

vice. Data can be stored in the device. either from inputs IN and IN; or from 1N and m. For example, if 1N and TF are both equal to zero-while. figflx, IN x, and simultaneously W: l, the binary digit x will be stored as a bistable set of the device. The operation of the circuit ofFIG. 6 may be characterized'by the Boolean expression: W(IN;-l-IN wherein stands for the logical operation or.- V In the FIG. 7 circuits, somewhat more'complexthresh old logic is performed at the input of the bistable device.

Heme, 1N IN Il\ and 'iT 'are data inputs and W and W are control inputs. If W and W are both zero. then the data inputs canhave any value Withoutsetting the.

bistable circuit. However data can be stored from either 1N and TN; or from 1N and Til To store data from IN and IN], W is set equal to one,,and W is set equal to one, while 1N and 1N are set equal-to zero. Then a signal inputs, IN =xi and IT I=5, Willbe set into the bistable circuit. To store data from IN; and I NQW is] set equal to one, and W is set equal to zero, while,

' IN =x, EV=5. In the latter instance 1N and IN; can have any arbitrary values.

FIG. 7 device may be described by the Boolean .expres- I sion i FIGS. 8, 9 and 10 are complete schematic diagrams of.

The operation of the bistable devices of FIGS. 5,6 and 7, respectively. Since. the circuit details are variations upon:the circuit illustrated and described in connection with FIG. 3, these circuits will not be completelyredescribed. Like reference numbers refer to like components, with the principal changes being herein noted. The primed numbers are applied to components of the second gate of the pair, or the gate receiving complemented inputs. Input resistors 1 1 determine by their paralleled resistances the :weight given. the. various .inputs; in some cases resistors are shunted'employing capacitors 32 in order to improve'the rise-time response of an input circuit." Each feedback connection includes a resistor 35 having an appropriate fraction of the resistor 1 value in order to give the feedback the proper specified weight. Thus in FIGQS, re.-

sistors 35 will have a resistance approximately one-third that of resistor 1 while in the circuit of FIG. 9, the feeds back resistors 35 have a resistance one-fourth that of re,-

sistor 1. In FIG. 10, feedback resistors are one-fifth the resistor 1 value.

Bistable devices in accordance with the present invention are applicable to form registers. for storing a com plete binary number, from a binary digit of a low order to that of a higher order digit position. These bistable devices are also applicable in more complex circuits, for example, the adding accumulator set forth and claimed in our copending application Serial Number 322,285, filed November 8, 1963 and entitled Accumulator. In the adding accumulator, an input number is added in parallel to the number stored in the accumulator, after which the resultant is placed in storage. The accumulator storage is changed only to the extent the digits of the new sum difier from those previously stored. Control inputs applied to the accumulator cause the accumulator to provide other functions, e.g. those of complementing, and shifting along the register.

While we have shown and described several embodiments of our invention, it will be apparent. to those skilled in the art that many changes and modifications may be made without departing from our invention in its broader aspects; and We therefore intend the appended claims to cover all such changes and modifications as fall within the true spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A bistable device comprising a pair of complementary threshold gates, each said gate having a plurality of inputs and generating an inverted output signal when the summation of input signals applied thereto exceeds a predetermined threshold value, cross-connections coupling the output of a first said threshold gate as an input to a second gate as well as coupling the output of said second gate as an input to said first gate, and means to provide the cross-connected inputs with weights respectively sufficient by themselves to supply signals which exceed the threshold value of said gates, and means coupling a single input to each of said gatesin parallel so that energization of said single input concurrently with another input to said first gate results in producing a binary zero output signal from said first gate and a binary one output signal from said second gate.

2. A bistable device comprising a pair of complementary threshold gates, each said gate having a plurality of input connections and generating an inverted output signal when the summation of input signals applied thereto exceeds a predetermined threshold value, a cross-connection coupling the output of a first said threshold gate to provide an input to the other gate, a cross-connection coupling the output of said other gate to provide an input to said first gate, means to provide the cross-coupled inputs with weights respectively suflicient by themselves to supply signals which exceed the threshold value of said gates to render the device bistable whereby one gate of the pair provides an output signal, and means coupling complementary input signals to said gates so that the bistable device will change its stable state when the input signals on the gate initially providing an output signal are jointly such as to exceed the threshold value of said gate initially providing an output while the input signals on the remaining gate are jointly such as not to exceed the threshold value of said remaining gate.

3. A bistable device comprising a first binary threshold gate and a second binary threshold gate, each said gate including a plurality of inputs and providing an output signal when the summation of input signal values applied thereto exceeds a predetermined threshold value, first means coupling the output of said first gate as an input of the second gate, second means coupling the output of the second gate as an input of the first gate, and means providing control signals to both said gates in conjunction with input signal values to exceed the threshold value of said first gate for entering a function of input signal into said device, said first and second means applying a weight to said first and second gate outputs respectively sufiicient to exceed the threshold value of said second and first gates, but the remaining input values being individually insufficient for exceeding the threshold value of said gates.

4. A bistable device comprising a first threshold gate and a second threshold gate, each said gate including plural logic input connections, means providing differing weights to said logic input connections, means providing further input connections to said gates for supplying con trol signal inputs coincident with said logic signal inputs to operate said gates to provide an output when the summation of weighted signal inputs exceeds a predetermined threshold value, first means coupling the output of the first gate to an input connection of the second gate, and second means coupling the output of thesecond gate to an input connection of the first gate, the weighted input signal provided by either said first or second means being suflicient to exceed the threshold value of said second or first gate respectively, but the other of said signals applied to input connections of each gate being insuflicient to singly exceed the threshold value thereof.

5. A bistable device comprising a first complementing threshold gate and a second complementing threshold gate, each said gate including logic input connections,

' means providing diflering input weights to said input connections, means providing control input signals identically to each of said gates for operating said gates to provide an output when the summation of weighted input signals exceeds a predetermined threshold value, the threshold References Cited by the Examiner UNITED STATES PATENTS 2,985,773 5/1961 Dobbie. 3,040,187 6/1962 Dobbie. 3,052,801 9/ 1962 Kaufman et al. 3,078,376 2/ 1963 Lewin. 3,093,751 6/1963 Williamson. 3,104,327 9/1963 Rowe. 3,110,821 11/1963 Webb. 3,153,200 10/ 1964 Wahrmen et al. 3,173,028 3/ 1965 Philips et a1. 3,178,590 4/1965 Heilweil.

OTHER REFERENCES Hurley: Junction Transistor Electronics, Wiley & Sons, 1958, PP. 390-398.

Rowe: The Transistor NOR Circuit, 1957 I.R.E. Wescon Record, Part 4, Aug. 23, 1957, pp. 231-245.

Mathias: Static Switching Devices, Control Engineering, May, 1957, p. 82 relied on.

Wolman et al.: Universal Boards Cut Computer Lead Time, Electronics, July 15, 1960, vol. 33, No. 29, p. 90.

ARTHUR GAUSS, Primary Examiner.

I. C. EDELL, Assistant Examiner. 

1. A BISTABLE DEVICE COMPRISING A PAIR OF COMPLEMENTRAY THRESHOLD GATES, EACH SAID GATE HAVING A PLURALITY OF INPUTS AND GENERATING AN INVERTED OUTPUT SIGNAL WHEN THE SUMMATION OF INPUT SIGNALS APPLIED THERETO EXCEEDS A PREDETERMINED THRESHOLD VALUE, CROSS-CONNECTIONS COUPLING THE OUTPUT OF A FIRST SAID THRESHOLD GATE AS AN INPUT TO A SECOND GATE AS WELL AS COUPLING THE OUTPUT OF SAID SECOND GATE AS AN INPUT TO SAID FIRST GATE, AND MEANS TO PROVIDE THE CROSS-CONNECTED INPUTS WITH WEIGHTS RESPECTIVELY SUFFICIENT BY THEMSELVES TO SUPPLY SIGNALS WHICH EXCEED THE THRESHOLD VALUE OF SAID GATES, AND MEANS COUPLING A SINGLE INPUT TO EACH OF SAID GATES IN PARALLEL SO THAT ENERGIZATION OF SAID SINGLE INPUT CONCURRENTLY WITH ANOTHER INPUT TO SAID FIRST GATE RESULTS IN PRODUCING A BINARY ZERO OUTPUT SIGNAL FROM SAID FIRST GATE AND A BINARY ONE OUTPUT SIGNAL FROM SAID SECOND GATE. 